Motor motion alters the distribution of crosslink extension \(r_c\) 
(Fig. REF b). The minimum \(r_c \approx 1\)  due to MT steric
interactions. For anti-aligned (polar-aligned) pairs, crosslink
relaxation shifts the distribution toward larger (smaller) extension.
The bulk material stress tensor is \({\bf \Sigma }_b = {{N k_{B} T} \over {V}} {\bf I} + {{1} \over {V}} \left\langle {\sum _i^N{\bf W}_i}\right\rangle \)  for \(N\)  interacting MTs in a volume \(V\) , with \({\bf W}_i = {1 \over 2} \sum _{j \ne i}^N {\bf r}_{ij} {\bf F}_{ij}\)  the
single-MT virial tensor [1]}, [2]}. Over a wide range of
motor parameters, the time-averaged bulk stress tensor \(\mathbf {\Sigma }_b\)  is
anisotropic, with larger components in the average MT alignment
direction. Denoting the alignment direction by \({\hat{{\bf y}}}\) , the stress
difference \(\Sigma _b^{yy}-\Sigma _b^{xx}\)  is positive, which
corresponds to an extensile stress. The stress difference can be
expressed as a sum of pair interactions, with each \(ij\)  pair
contributing a stresslet \(S_{ij}\) , prior to division by the bulk
volume. The average pair stresslet \(S\)  (green symbols in
Fig. REF c) increases with the motor run length \(\ell \)  up to a
maximum when the typical motor run length is the MT length. Here
\(\ell = v_m/k_0 l\) , the typical distance a motor travels during one
binding event, with \(k_0\)  a base binding rate of motors. Increasing
\(\ell \)  further leads to decreasing \(S\)  because the motors rapidly move
to the ends of the MTs and unbind.
